Is your feature request related to a problem? Please describe.

A huge value-add to using replacements over traditional JSON patches is the ability to use [key=value] selectors in field paths. This makes your patches much more robust since the alternative relies on array field ordering. Unfortunately, this is still true when the field path value is a simple string array... most notably in the case of container args.

Describe the solution you'd like

In situations like the following, it would be ideal if we could reliably replace the value of --leader-election-namespace:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: cert-manager
spec:
  template:
    spec:
      containers:
      - name: cert-manager
        image: quay.io/jetstack/cert-manager-controller:v1.2.0
        args:
        - --v=2
        - --default-issuer-kind=ClusterIssuer
        - --leader-election-namespace=kube-system
# ...
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: cert-manager-cainjector
spec:
  template:
    spec:
      containers:
      - name: cert-manager
        image: quay.io/jetstack/cert-manager-cainjector:v1.2.0
        args:
        - --v=2
        - --leader-election-namespace=kube-system
# ...

I'm not sure how we would represent this in the replacements API, but it could look something like:

replacements:
- source:
    kind: Role
    name: cert-manager-cainjector:leaderelection
    fieldPath: metadata.namespace
  targets:
  - select:
      kind: Deployment
    fieldPaths:
    - spec.template.spec.containers.[name=cert-manager].args
    options:
      prefix: "--leader-election-namespace="

The new rules would be:

• index and delimiter options can only be specified on scalar values and lists consisting only of string values
• when prefix and/or suffix options are specified,
  • the value is only replaced if it contains the specified prefix and suffix
  • the matched prefix and/or suffix is preserved in the resulting value
  • when used in conjunction with delimiter, only the unprefixed/suffixed portion of the string is split